Acutiramus is a genus of giant predatory eurypterid, an extinct group of aquatic arthropods. Fossils of Acutiramus have been discovered in deposits of Late Silurian to Early Devonian age. Eight species have been described, five from North America (including A. cummingsi, the type species) and two from the Czech Republic (with one of them potentially found in Australia as well). The generic name derives from Latin acuto ("acute" or "sharp") and Latin ramus ("branch"), referring to the acute angle of the final tooth of the claws relative to the rest of the claw.

Acutiramus is classified as part of the eurypterid family Pterygotidae, a family differentiated from other eurypterids by their flattened telsons (the most posterior segment of the body) and their modified chelicerae (frontal appendages), ending in well-developed chelae (claws). Many pterygotid eurypterids, such as Jaekelopterus and Pterygotus, grew to gigantic proportions. Acutiramus was no exception, with the largest species, A. bohemicus, surpassing 2 metres (6.6 feet) in length. All species were not this large though, with some (such as A. floweri) only reaching lengths of 20 centimetres (7.9 in).

Other giant pterygotid eurypterids, such as Jaekelopterus and Pterygotus, were very visually acute (possessed good and sharp eyesight) in a similar way to modern predatory arthropods and possessed robust and crushing chelae. Studies on the compound eyes and chelicerae of Acutiramus has revealed that it would have had a comparably low visual acuity and claws adapted for slicing and shearing, rather than crushing. This suggests that the ecological role of Acutiramus was distinct from that of other pterygotids, it potentially lived a lifestyle of ambush predation or scavenging on soft-bodied animals, feeding during the night or in otherwise low-light conditions.

Acutiramus was one of the largest eurypterids, perhaps only smaller than the gigantic Jaekelopterus rhenaniae. At a length of 2.1 metres (6.9 feet), A. bohemicus is the largest known species of the genus, whilst the smallest were A. floweri and A. perryensis both at a length of 20 cm (7.9 in). The body of Acutiramus was very slender, with members of the genus being almost five times as long as they were wide.

Acutiramus can be distinguished from other pterygotids by the distal margin of the chelae (pincers), where the final tooth is at an acute angle relative to the rest of the claw. The large tooth in the center of the claw is distally inclined (points forwards). The prosoma (head) is subquadrate (nearly square-shaped), with compound eyes located at the edge of the front corners. The telson (the posteriormost segment of its body) has a low row of knobs running down its center.

Acutiramus is classified as part of the pterygotid family of eurypterids, a group of highly derived eurypterids of the Silurian to Devonian periods that differ from other groups by a number of features, perhaps most prominently in the chelicerae (the first pair of limbs) and the telson. The chelicerae of the Pterygotidae were enlarged and robust, clearly adapted to be used for active prey capture and more similar to the claws of some modern crustaceans, with well-developed teeth on the claws, than to the chelicerae of other eurypterid groups. Another feature distinguishing the group from other eurypterid groups were their flattened and expanded telsons, likely used as rudders when swimming. Their walking legs were small and slender, without spines, and they were likely not capable of walking on land.

A. bohemicus was once estimated to have reached lengths of 2.3 to 2.5 metres (7.5 to 8.2 ft) based on the largest known chelicerae and coxae. This would have made it the largest known arthropod, but recent estimates have revised the upper size limits to 2.1 metres (6.9 feet). A. bohemicus is surpassed in size among the eurypterids only by Jaekelopterus rhenaniae, which measured 2.5 metres (8.2 feet) in length. Likewise, A. cummingsi was also once thought to have been the largest eurypterid (before the discovery of larger individuals in Acutiramus and Jaekelopterus), with fragmentary fossils suggesting great lengths. The largest preserved tergite suggests that the species could grow up to 1.65 metres in length, while a gnathobase (a lower appendage used to macerate, or soften and cut, food) from Buffalo was estimated to have belonged to a specimen reaching 2 metres (6.6 feet) in length. A. macropthalmus was also large in size, with fragmentary tergites from Litchfield, New York suggesting sizes in the range of 1.65 to 2 metres (5.4 to 6.6 ft).

The earliest species of Acutiramus to be named was A. macropthalmus (as a species of Pterygotus, Pterygotus macrophthalmus) in 1859. The type specimen, the carapace of a young individual, was discovered in waterlime deposits of Upper Silurian age in Litchfield, New York. At virtually the same time, the type specimen of another species of Pterygotus, P. osborni, was described from the same locality, this specimen preserving the prosoma and four abdominal segments but not the carapace. Later more complete remains from the locality allowed the two species to be synonymized in 1912 by John Mason Clarke and Rudolf Ruedemann as the fossils of P. osborni proved identical to more complete remains referred to P. marcophthalmus.